Surface acoustic wave device and manufacturing method therefor

ABSTRACT

A surface acoustic wave device has superior electrical power resistance that is obtained by improving stress migration resistance of electrodes. In order to form at least one electrode, for example, on a θ rotation Y-cut (θ=36° to 42°) LiTaO 3  piezoelectric substrate, an underlying electrode layer including Ti or Cr as a primary component is formed, and an Al electrode layer including Al as a primary component is then formed on this underlying electrode layer. The Al electrode layer is an oriented film grown by epitaxial growth and is also a polycrystalline thin film having a twin structure in which a diffraction pattern observed in an X-ray diffraction pole figure has a plurality of symmetry centers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surface acoustic wave devices, such assurface acoustic wave resonators or surface acoustic wave filters, andmanufacturing methods therefor, and more particularly, relates to anelectrode structure of a surface acoustic wave device and a formingmethod therefor.

2. Description of the Related Art

As has been well known, surface acoustic wave devices are electronicelements including a surface acoustic wave in which mechanical vibrationenergy is concentrated only in the vicinity of surfaces of a solidmaterial and is then propagated. In addition, the surface acoustic wavedevices are each generally composed of a piezoelectric substrate havingpiezoelectric properties and electrodes, such as interdigital electrodesand/or grating electrodes, disposed on the piezoelectric substrate forprocessing electrical signals and surface acoustic waves.

In the surface acoustic wave devices described above, as an electrodematerial, aluminum (Al) having a low electrical resistivity and a lowspecific gravity or an alloy thereof has been used.

However, since Al has poor stress migration resistance, when a largeelectrical power is applied thereto, hillocks and/or voids are formed inthe electrodes, and short-circuiting or disconnection of the electrodesmay occur in some cases, resulting in breakage of the surface acousticwave device.

In order to solve the problems described above, a method for improvingelectrical power resistance has been disclosed in Japanese UnexaminedPatent Application Publication No. 7-162255 (patent publication 1) inwhich the crystal orientation is improved by an ion beam sputteringmethod used as a method for forming electrodes.

In addition, another method for improving electrical power resistancehas been proposed in Japanese Unexamined Patent Application PublicationNo. 3-48511 (patent publication 2) in which an Al crystal is oriented ina predetermined direction by an epitaxial growth method.

Japanese Unexamined Patent Application Publication No. 6-6173 (patentpublication 3) has disclosed that electrical power resistance ofelectrodes can be improved as crystal grain size is decreased.

Furthermore, in “Technical Handbook of Surface Acoustic Wave Device”edited by the 150th Committee on Technology of Surface Acoustic WaveDevice of Japan Society for the Promotion of Science, published byOhmsha, Ltd., p. 267 (non-patent publication 1), a phenomenon has beendisclosed in which the electrical power resistance is improved whencopper (Cu) is added to Al.

Patent publication 1: Japanese Unexamined Patent Application PublicationNo. 7-162255

Patent publication 2: Japanese Unexamined Patent Application PublicationNo. 3-48511

Patent publication 3: Japanese Unexamined Patent Application PublicationNo. 6-6173

Non-patent publication 1: “Technical Handbook of Surface Acoustic WaveDevice” edited by the 150th Committee on Technology of Surface AcousticWave Device of Japan Society for the Promotion of Science, published byOhmsha, Ltd., p. 267.

However, by the traditional techniques disclosed in patent publications1 and 3, recent higher frequency and larger electrical powerrequirements cannot satisfactorily be fulfilled, and hence, when thetechniques described above are used in high-frequency or largeelectrical power applications, insufficient electrical power resistancebecomes a serious problem.

In addition, according to the traditional technique disclosed in patentpublication 2, an epitaxial film having superior crystallinity can beactually grown only on a quartz substrate. However, on a substratecomposed of a piezoelectric crystal, such as LiTaO₃ or LiNbO₃, used forfilters which have superior piezoelectric properties and areadvantageously used in a broad band, it has been difficult to grow anepitaxial film having superior crystallinity by the technique disclosedin patent publication 2, and as a result, the traditional techniquedescribed above cannot practically be applied to a surface acoustic wavedevice including a LiTaO₃ or LiNbO₃ substrate.

According to the traditional technique disclosed in non-patentpublication 1, by adding Cu to Al, the electrical power resistance canactually be improved. However, a level of this improvement has not beensatisfactory in practice.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a surface acoustic wave device and amanufacturing method therefore, in which the novel surface acoustic wavedevice achieves superior electrical power resistance by using anepitaxial Al film having a twin structure for an Al electrode layerwhich is primarily composed of Al and which constitutes an electrodeprovided on a piezoelectric substrate formed, for example, of a 64° Y-Xcut LiNbO₃. In this case, it was understood that the epitaxial Al filmgrows in a particular manner, that is, the (111) plane thereof isoriented with respect to a Z axis of the piezoelectric substrate and hasa twin structure which is grown in the (111) plane.

Compared to a single crystal, mechanical strength of an epitaxial filmhaving a twin structure is very high, and as a result, the plasticdeformation is unlikely to occur. Accordingly, a significant advantagecan be obtained in that electrode breakage of surface acoustic wavedevices, which is frequently caused by stress migration, is preventedfrom occurring.

Through intensive research by the inventors of the present inventionregarding the epitaxial Al film having the twin structure describedabove, it was understood that, in some cases, crystal growth may occuraccording to a mechanism which is totally different from that in whichthe epitaxial film grows while the (111) plane of the Al film isoriented with respect to the Z axis as described above. In this case,the Al(111) plane is not oriented along the Z axis of the piezoelectricsubstrate, and very particular crystal growth occurs in which theAl(111) are oriented in a plurality of directions. The crystal growthdescribed above is observed in particular when a Y-cut piezoelectricsingle crystal is used as a piezoelectric substrate, and in moreparticular, when 36° to 42° Y-cut LiTaO₃ substrate is used. Othersuitable substrates may also be used.

According to the information thus obtained, a preferred embodiment ofthe present invention provides a surface acoustic wave device includinga piezoelectric substrate made of a piezoelectric single crystal and atleast one electrode provided on the piezoelectric substrate, and the atleast one electrode has an electrode layer which is an orientedelectrode layer formed by epitaxial growth, and the electrode layer is apolycrystalline thin film having a twin structure in which a diffractionpattern observed in an X-ray diffraction pole figure has a plurality ofsymmetry centers.

The electrode layer described above preferably includes Al as a primarycomponent.

The electrode described above may further include an underlyingelectrode layer provided between the electrode layer and thepiezoelectric substrate for improving the crystallinity of the electrodelayer. This underlying electrode layer may include at least one oftitanium (Ti) and chromium (Cr) as a primary component.

In addition, the electrode may further include an intermediate electrodelayer provided between the Al electrode layer and the underlyingelectrode layer so as to cause a crystal face present the surface of theunderlying layer to be in a cleaner state.

The piezoelectric substrate preferably includes a LiNbO₃ or a LiTaO₃single crystal and, more preferably, is a θ rotation Y-cut (θ is between36° and 42°) LiTaO₃ substrate.

Concerning the crystal orientation of the electrode layer provided forthe surface acoustic wave device of preferred embodiments of the presentinvention, in X-ray diffraction in which X-rays are incident on the(200) plane of the crystal constituting the electrode layer, the [111]direction of the crystal is preferably oriented so as to approximatelycoincide with the center of symmetry spots detected in the X-raydiffraction pole figure.

In addition, the symmetry spots in the X-ray diffraction pole figurepreferably have at least two centers, the crystal of the electrode layermay grow in at least two [111] directions, and the [111] directions ofthe crystal may be oriented so as to approximately coincide with thecenters of the symmetry spots detected in the X-ray diffraction polefigure.

In the case described above, the symmetry spots detected in the X-raydiffraction may form three-fold or six-fold symmetry.

According to another preferred embodiment of the present invention, amethod for manufacturing a surface acoustic wave device including apiezoelectric substrate formed of a Y-cut piezoelectric single crystal,and at least one electrode formed on the piezoelectric substrate, theelectrode including an Al electrode layer primarily composed of Al andan underlying electrode layer provided between the piezoelectricsubstrate and the Al electrode layer for improving the crystallinitythereof, the Al electrode layer being an oriented film formed byepitaxial growth and being a polycrystalline thin film having a twinstructure in which a diffraction pattern observed in an X-raydiffraction pole figure has a plurality of symmetry centers, isprovided.

The method for manufacturing the surface acoustic wave device of apreferred embodiment of the present invention includes preparing thepiezoelectric substrate formed of the Y-cut piezoelectric singlecrystal, forming the underlying electrode layer on the piezoelectricsubstrate, forming the Al electrode layer on the underlying electrodelayer, and performing etching treatment for the piezoelectric substrateprior to the step of forming the underlying electrode layer to expose acrystal face on a surface of the piezoelectric substrate so that the Alelectrode layer can be formed by epitaxial growth.

The etching step described above is preferably performed using anetchant including at least one selected from the group consisting ofphosphoric acid, pyrophosphoric acid, benzoic acid, octanoic acid,hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid,buffered hydrofluoric acid (BHF), and potassium hydrogen sulfate.

In addition, through further detailed investigation regarding theepitaxial Al film having a twin structure, it was understood thatcrystal growth may occur in some cases according to a mechanism which istotally different from the mechanism described above in which theepitaxial growth proceeds while the (111) plane of the Al film isoriented with respect to the Z axis. The crystal growth described aboveis observed in particular when a low cut angle substrate, such as a 36°Y-cut piezoelectric single crystal, is used as a piezoelectricsubstrate, and is very particular crystal growth in which the Al(111)planes are oriented in at least two directions which are different fromthe Z axis of the piezoelectric substrate.

Depending on process conditions in which the underlying electrode layerand/or the Al electrode layer is formed, the crystal growth describedabove may not be performed in some cases. Through further detailedinvestigation on the phenomenon described above, it was understood thatthe Al electrode layer including an epitaxial Al film having a twinstructure cannot be obtained unless the underlying electrode layer isformed using Ti by heating to a temperature of 70° C. or more, and theAl electrode layer is formed at a relatively low temperature of 50° C.or less. The reason for this is that when the Al electrode layer isformed by heating, due to the counter diffusion between Al and Ti,epitaxial growth of Al is inhibited.

A surface acoustic wave device produced by a method according to apreferred embodiment of the present invention includes a piezoelectricsubstrate, and at least one electrode formed on the piezoelectricsubstrate, the at least one electrode including an Al electrode layerprimarily composed of Al and an underlying electrode layer providedbetween the piezoelectric substrate and the Al electrode layer forimproving the crystallinity thereof. The method according to thispreferred embodiment includes the following steps.

That is, the method includes a step of preparing the piezoelectricsubstrate, a step of forming the underlying electrode layer on thepiezoelectric substrate by heating to a temperature of about 70° C. ormore, and a subsequent step of forming the Al electrode layer at arelatively low temperature of about 50° C. or less.

The step of forming the underlying electrode layer by heating ispreferably performed at a temperature of about 300° C. or less.

In addition, the step of forming the Al electrode layer at a relativelylow temperature is preferably performed at a temperature of about 0° C.or more.

In preferred embodiments of the present invention, the piezoelectricsubstrate may include a Y-cut piezoelectric single crystal. In thiscase, the piezoelectric substrate is preferably a LiNbO₃ or a LiTaO₃single crystal and is more preferably a θ rotation Y-cut (θ=36° to 42°)LiTaO₃ substrate.

In the step of forming the Al electrode layer, the Al electrode layer ispreferably grown so as to form an epitaxial film having a twinstructure.

In addition, prior to the step of forming the underlying electrodelayer, the present invention may further include a step of performingpretreatment for the piezoelectric substrate to expose a crystal face ona surface thereof so that the Al electrode layer can be formed byepitaxial growth. The underlying electrode layer preferably includes atleast one of Ti and Cr as a primary component.

The present invention may further include a step of forming anintermediate electrode layer on the underlying electrode layer at a lowtemperature of about 50° C. or less for placing a crystal face presenton the underlying electrode layer in a cleaner state, wherein the Alelectrode layer is preferably formed on the intermediate electrode layerin the step of forming the Al electrode layer.

The intermediate electrode layer preferably includes at least one of Tiand Cr as a primary component or preferably includes the same materialas that for the underlying electrode layer.

In addition, the step of forming the intermediate electrode layer at alow temperature is preferably performed at a temperature of about 0° C.or more.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly cross-sectional view of a surface acoustic wavedevice of a preferred embodiment according to the present invention;

FIG. 2 is a view showing an X-ray diffraction pole figure of an Alelectrode layer obtained in a first example according to a preferredembodiment the present invention;

FIG. 3 is a view showing the X-ray diffraction pole shown in FIG. 2provided with additional lines for illustrating the diffraction pattern;

FIG. 4 is a view showing an X-ray diffraction pole figure of an Alelectrode layer obtained in a second example according to preferredembodiments of the present invention;

FIG. 5 is a view showing the X-ray diffraction pole shown in FIG. 4provided with additional lines for illustrating the diffraction pattern;

FIG. 6 is a view showing the X-ray diffraction pole shown in FIG. 4provided with additional lines different from those in FIG. 5 forillustrating the diffraction pattern;

FIG. 7 is a view showing the relationship between the Ti film-formingtemperature for forming an underlying electrode layer and spot intensityof an X-ray diffraction pole figure of an Al electrode layer when X-raysare incident on the Al (200); and

FIG. 8 is a partly cross-sectional view of a surface acoustic wavedevice formed by a manufacturing method of another preferred embodimentaccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a partly cross-sectional view of a surface acoustic wavedevice 1 of a preferred embodiment according to the present invention,and in the figure, an electrode 3 disposed on a piezoelectric substrate2 is shown.

The piezoelectric substrate 2 is preferably made of a piezoelectricsingle crystal, such as a Y-cut LiTaO₃ or LiNbO₃ single crystal. Thepiezoelectric substrate 2 is preferably composed of a θ rotationY-cut(θ=36° to 42°) LiTaO₃ substrate.

The electrode 3 includes an Al electrode layer 4 preferably made of Alor an Al alloy primarily composed of Al. Between the Al electrode layer4 and the piezoelectric substrate 2, an underlying electrode layer 5 ispreferably provided for improving the crystallinity of the Al electrodelayer 4. The underlying electrode layer 5 is primarily composed, forexample, of at least one of Ti and Cr.

In order to manufacture the surface acoustic wave device 1, thefollowing steps are performed, that is, the piezoelectric substrate 2 isprepared, the underlying electrode layer 5 is formed on thepiezoelectric substrate 2, and subsequently, the Al electrode layer 4 isformed on the underlying electrode layer 5. Furthermore, the electrode 3is formed into an interdigital shape by a photolithographic techniqueand a dry etching technique.

The Al electrode layer 4 is an oriented film formed by epitaxial growthand is a polycrystalline thin film having a twin structure in which adiffraction pattern observed in an X-ray diffraction pole figure has aplurality of symmetry centers.

In order to form the Al electrode layer 4 having the particular crystalstructure described above, when the surface acoustic wave device 1 ismanufactured, before the step of forming the underlying electrode layer5 is performed, etching treatment is performed on the piezoelectricsubstrate 2 to expose a crystal face on the surface thereof so that theAl electrode layer 4 can be formed by epitaxial growth.

As an etchant used in the etching treatment described above, solventscontaining phosphoric acid, pyrophosphoric acid, benzoic acid, octanoicacid, hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid,buffered hydrofluoric acid (BHF), and potassium hydrogen sulfate may bepreferably used alone or in combination.

According to the etching treatment described above, since a deterioratedlayer having a thickness of several nanometers formed on the surface ofthe piezoelectric substrate 2 by machining, such as polishing, isremoved, the crystal surface is exposed on the surface of thepiezoelectric substrate 2, and hence information on crystal alignment,which is necessary for epitaxial growth, can be transmitted to the Alelectrode layer 4.

In general, it has been believed that the presence of crystal grainboundaries of an Al electrode layer degrades the electrical powerresistance of a surface acoustic wave device. The reason for this isthat self-diffusion of Al occurs through the crystal grain boundaries bystress migration, resulting in the formation of defects such asso-called hillocks and/or voids. However, in the polycrystalline Alelectrode layer 4 obtained according to preferred embodiments of thepresent invention, the thickness of the crystal grain boundary is oneatomic distance or less, and hence the self-diffusion through thiscrystal grain boundary does not occur.

A polycrystalline metal has a mechanical strength that is higher thanthat of a single crystal metal. The reason for this is because of theplastic deformation mechanism of metals. That is, as the plasticdeformation, shear deformation of a crystal occurs when an externalforce (vibration by the piezoelectric effect in the field of surfaceacoustic wave devices) is applied. Accordingly, the shear deformation ina single crystal occurs only by the activity of the most movable shearsystem therein, and on the other hand, the shear deformation in a polycrystal occurs as a result of the activities of a plurality of shearsystems (see “Metal Handbook”, fifth edition, Maruzen Co., Ltd., pp. 337to 343). Related to those described above, the resistance againstplastic deformation relates to the resistance against electrode breakagecaused by stress migration, and accordingly, an electrode structurehaving small grain diameters tends to have superior electrical powerresistance.

As has been described, when the Al electrode layer 4 is an oriented filmhaving the twin structure, the effect of preventing the formation ofhillocks and/or voids, which are formed by the self-diffusion ofelectrode-forming atoms through the crystal grain boundaries, andsuperior electrical power resistance because of the resistance againstthe plastic deformation can be simultaneously obtained.

As described in non-patent publication 1, it has been well known that,by adding a different type of metal such as Cu to the Al electrode layer4, the formation of hillocks and/or voids can be suppressed, and thatthe electrical power resistance can be improved. Accordingly, in the Alelectrode layer 4, in addition to the use of an epitaxial Al film havingthe twin structure, when addition of Cu or the like is performed, theelectrical power resistance can be further improved. As the additiveshaving the effect of improving the electrical power resistance, inaddition to Cu, for example, magnesium (Mg), nickel (Ni), and molybdenum(Mo) may be used. Hence, when the Al electrode layer 4 is formed of anAl alloy containing a small amount of at least one of these additives,the electrical power resistance can be further improved.

Although not shown in FIG. 1, a thin insulating film may be formed so asto cover the upper surface and the side surfaces of the electrode 3.

Hereinafter, particular examples of the surface acoustic wave deviceaccording to preferred embodiments of the present invention and themanufacturing method therefor will be described.

FIRST EXAMPLE

In order to form a surface acoustic wave device of a first exampleaccording to preferred embodiments of the present invention, apiezoelectric substrate 2 composed of a 36° Y-cut LiTaO₃ single crystalwas first prepared, and then by performing pretreatment for thispiezoelectric substrate 2 using a buffered hydrofluoric acid (BHF)solution at room temperature for 10 minutes, a deteriorated surfacelayer, which was present on the surface of the piezoelectric substrate 2and inhibited epitaxial growth, was removed.

Next, by an electron beam deposition method, an underlying electrodelayer 5 was formed at a deposition temperature of about 180° C. using Tiso as to have a thickness of about 20 nm and was then cooled to roomtemperature in an evacuated state.

After the cooling mentioned above, an Al electrode layer 4 was formedusing Al so as to have a thickness of about 100 nm.

X-ray diffraction analysis was performed for the Al electrode layer 4thus formed. FIG. 2 shows an X-ray diffraction pole figure obtained bythis analysis. This X-ray diffraction pole figure was obtained whenX-rays were incident on the (200) plane of the Al electrode layer 4. Asshown in FIG. 3, additional lines were drawn for illustrating thediffraction pattern shown in FIG. 2.

As shown in FIGS. 2 and 3, the Al electrode layer 4 was a thin epitaxialfilm in which diffraction spots having a plurality of symmetry spotswere observed in the X-ray diffraction pole figure when X-rays wereincident on the (200) plane of the Al. Six spots in FIGS. 2 and 3 showthe detection of reflection signals from the (200) plane of the Al.

As can be seen from FIG. 3, the six diffraction spots thus detected weretwo sets of three-fold symmetry spots. The angular distance between eachspot and the corresponding symmetry center in the ψ direction wasapproximately 55°, and each symmetry center approximately coincided withthe (111) direction of Al. In addition, the symmetry center and thenormal line (the center of the pole figure) of the piezoelectricsubstrate 2 were spaced apart from each other by approximately ±10 to20° in the ψ direction. In FIG. 3, the distance between the symmetrycenter and the center of the pole figure was approximately 17° in the ydirection. However, since this angle varies in accordance withconditions of the piezoelectric substrate 2, a film-forming temperature,or the like, in consideration of this variation, the distance can beconsidered to be in the range of approximately ±10 to 20° as describedabove.

In the pole figure in the case of X-rays being incident on the (200)plane of the Al, the fact that the three-fold symmetry spots wereobserved means that the [111] direction of Al was oriented so as toapproximately coincide with the center of the symmetry spots, that is, atriaxial orientation film was formed in which the Al [111] axis wasgrown in the direction toward the symmetry center.

In addition, the fact that the two sets of the three-fold symmetry spotswere observed means that the two centers of the symmetry spots werepresent, and that the Al[111]-oriented crystal had two growingdirections. In other words, the [111]-oriented single-crystal Al grew intwo orientation directions, that is, the twin structure was formed.

As described above, it was confirmed that the film forming the Alelectrode layer 4 is a triaxial-orientation epitaxial film formed of theAl(111) oriented in two growing directions and is also a polycrystallinefilm having the twin structure.

Next, the electrode 3 was formed into an interdigital shape using aphotolithographic technique and a dry etching technique, thereby formingthe surface acoustic wave filter defining the surface acoustic wavedevice 1.

As a comparative example for the first example described above, when aTi film was formed without heating for forming the underlying electrodelayer 5, an epitaxial film used as the Al electrode layer 4 could not beformed, and a uniaxial orientation film was formed in which the Al(111)plane grew perpendicular to the piezoelectric substrate 2.

When a constant electrical power was applied to each of the surfaceacoustic wave filters thus formed in order to compare the electricalpower resistances therebetween, a service life (time until failure) ofthe filter formed in the first example was 1,000 times or more than thatof the filter formed in the comparative example.

SECOND EXAMPLE

In a second example, a piezoelectric substrate 2 composed of a 42° Y-cutLiTaO₃ single crystal was prepared. Subsequently, in a processequivalent to that in the first example, the underlying electrode layer5 was formed, and the Al electrode layer 4 was formed thereon.

FIG. 4 shows an X-ray diffraction pole figure obtained in this secondexample in which X-rays were incident on the (200) plane of the Alelectrode layer 4. As shown in FIGS. 5 and 6, additional lines weredrawn for illustrating the diffraction pattern shown in FIG. 4.

As shown in FIGS. 4, 5, and 6, the Al electrode layer 4 was a thinepitaxial film in which diffraction spots having a plurality of symmetrycenters were observed. Twelve spots in FIGS. 4, 5 and 6 show thedetection of reflection signals from the (200) plane of the Al.

As can be apparently seen in FIG. 5, the twelve diffraction spots thusdetected were two sets of six-fold symmetry spots. As in the firstexample, the angular distance between each spot and the correspondingsymmetry center in the ψ direction was approximately 55°, and eachsymmetric center approximately coincided with the [111] direction of Al.In addition, the symmetry center and the normal line (the center of thepole figure) of the piezoelectric substrate 2 were spaced apart fromeach other by approximately ±10 to 20° in the ψ direction.

In the pole figure in the case of X-rays being incident on the (200)plane of the Al, the fact that the six-fold symmetry spots were observedmeans that a triaxial orientation film was formed in which theAl[111]-oriented film was grown in the direction toward the symmetrycenter.

In addition, the six-fold symmetry spots themselves mean the formationof a twin. The reason for this is that when X-rays are incident on theAl (200) plane, the diffraction spots from the Al(111) single crystalare detected as the three-fold symmetry spots each located at a positionapart from the symmetry center by approximately 55°. In more particular,there are three symmetry spots, i.e., (100), (010), and (001). That is,the six-fold symmetry spots are formed of two sets of three-foldsymmetry spots, as shown in FIG. 6, and these two sets of the three-foldsymmetry spots have the positional relationship with each other in whichone set is located at a position that is rotated by 180° from that ofthe other set.

As described above, in the second example, since the two sets of thesix-fold symmetry spots having the twin structure were present as shownin FIG. 5, this Al electrode layer 4 can also be regarded as “a twinformed of two twins” (four single domains are present). That is, when itis assumed that the two sets of the three-fold symmetry spots shown inFIG. 2 each indicate a twin, the film structure described above may beeasily understood.

Although the very complicated diffraction spots are shown in FIG. 4, theAl electrode layer 4 obtained in the second example is also a triaxialepitaxial film formed of the Al(111) oriented in two growing directionsand is essentially the same as that in the first example.

Next, the electrode 3 was formed into an interdigital shape using aphotolithographic technique and a dry etching technique, thereby forminga surface acoustic wave filter as the surface acoustic wave device 1. Inaddition, a surface acoustic wave filter having an Al electrode layer,which was a uniaxial orientation film, was also formed as a comparativeexample in which the Al(111) plane grew perpendicular to thepiezoelectric substrate 2. When the surface acoustic wave filters thusformed were compared to each other, the electrical power resistance ofthe surface acoustic wave filter formed in the second example was 1,000times or more that of the comparative example. That is, concerning theelectrical power resistance, it was understood that the advantage in thesecond example is equivalent to that in the first example.

THIRD EXAMPLE

In a third example, a surface acoustic wave filter was formed having thestructure shown in FIG. 1 as the surface acoustic wave device 1.

First, a piezoelectric substrate 2 composed of a 36° Y-cut LiTaO₃ singlecrystal was prepared.

Subsequently, on the piezoelectric substrate 2, the underlying electrodelayer 5 was formed by an electron beam deposition method using Ti at atemperature of about 70° C. so as to have a thickness of about 20 nm,and was then cooled to a temperature of about 50° C. or less in anevacuated state.

After the cooling described above, the Al electrode layer 4 was formedusing Al so as to have a thickness of about 100 nm.

The Al electrode layer 4 thus formed had a half value width of theAl(111) of approximately 2° measured by X-ray diffraction. As acomparative example, when the underlying electrode layer 5 and the Alelectrode layer 4 were formed without heating as described above, thehalf value width of the Al(111) thereof was approximately 5°. Accordingto the third example described above, it was confirmed thatsignificantly higher orientation can be obtained.

Next, the electrode 3 was formed into an interdigital shape using aphotolithographic technique and a dry etching technique, thereby formingthe surface acoustic wave filter defining the surface acoustic wavedevice 1.

When the electrical power resistances of the surface acoustic wavefilter of the third example and that of the comparative example werecompared to each other by applying a constant electrical power, aservice life of that in the third example was much longer than that inthe comparative example.

FOURTH EXAMPLE

In a fourth example, a surface acoustic wave filter was formed as asurface acoustic wave device 1 a having the structure shown in FIG. 8.

First, a piezoelectric substrate 2 composed of a 36° Y-cut LiTaO₃ singlecrystal was prepared.

Subsequently, on the piezoelectric substrate 2, the underlying electrodelayer 5 was formed by an electron beam deposition method using Ti at atemperature of about 70° C. so as to have a thickness of about 10 nm,and was then cooled to a temperature of about 50° C. or less in anevacuated state.

After the cooling described above, an intermediate electrode layer 6composed of Ti, which was the same material as that for the underlyingelectrode layer 5, was formed at a temperature of about 50° or less soas to have a thickness of about 10 nm.

The Al electrode layer 4 was then formed using Al so as to have athickness of about 100 nm.

The Al electrode layer 4 thus formed had a half value width of theAl(111) of approximately 1.8° measured by X-ray diffraction. Accordingto the fourth example, it was confirmed that a layer havingsignificantly high orientation can be obtained as compared to thoseobtained in the comparative example and the third example.

Next, the electrode 3 was formed into an interdigital shape using aphotolithographic technique and a dry etching technique, thereby formingthe surface acoustic wave filter as the surface acoustic wave device 1a.

When the electrical power resistances of this surface acoustic wavefilter of the fourth example was compared with those of the comparativeexample and the third example by applying a constant electrical power, aservice life of the filter in the fourth example was much longer ascompared to the other filters described above.

FIFTH EXAMPLE

In a fifth example, a surface acoustic wave filter was formed as thesurface acoustic wave device 1 a having the structure shown in FIG. 8.

First, a piezoelectric substrate 2 composed of a 36° Y-cut LiTaO₃ singlecrystal was prepared and was then processed by pretreatment using abuffered hydrogen fluoride (BHF) solution at room temperature for tenminutes, thereby removing a deteriorated layer which was present on thesurface of the piezoelectric substrate 2 and inhibited epitaxial growth.

Subsequently, the underlying electrode layer 5 was formed by an electronbeam deposition method using Ti at a temperature of about 180° C. so asto have a thickness of about 10 nm and was then cooled to roomtemperature in an evacuated state.

After the cooling described above, the intermediate electrode layer 6composed of Ti, which was the same material as that for the underlyingelectrode layer 5, was formed so as to have a thickness of about 10 nm.

The Al electrode layer 4 was then formed using Al so as to have athickness of about 100 nm.

X-ray diffraction analysis was performed for the Al electrode layer 4thus formed. The result obtained was equivalent to that shown in theX-ray diffraction pole figure obtained in the first example.

That is, as shown in FIG. 3, the Al electrode layer 4 was a thinepitaxial film in which six diffraction spots were observed in the X-raydiffraction pole figure obtained when X-rays were incident on theAl(200) plane. Since reflection signals from the Al(200) plane weredetected as the two sets of the three-fold symmetry spots, it wasconfirmed that the film forming the Al electrode layer 4 is a triaxialorientation film and is also a polycrystalline film having a twinstructure.

Next, the electrode 3 was formed into an interdigital shape using aphotolithographic technique and a dry etching technique, thereby formingthe surface acoustic wave filter defining the surface acoustic wavedevice 1 a.

As a comparative example for the fifth example, when a film was formedusing Ti without heating in order to form the underlying electrode layer5, an epitaxial film was not formed used as the Al electrode layer 4,and a uniaxial orientation film in which the Al(111) plane grewperpendicular to the piezoelectric substrate 2 was formed.

When the electrical power resistances of these surface acoustic wavefilters thus obtained were compared to each other by applying a constantelectrical power, a service life of the filter in the fifth example was1,000 times or more than that of the comparative example.

SIXTH EXAMPLE

In a sixth example, a surface acoustic wave filter was formed as thesurface acoustic wave device 1 a having the structure shown in FIG. 8.

First, a piezoelectric substrate 2 composed of a 42° Y-cut LiTaO₃ singlecrystal was prepared. Subsequently, by using a process equivalent tothat in the third example, the underlying electrode layer 5, theintermediate electrode layer 6, and the Al electrode layer 4 were formedon the piezoelectric substrate 2 in that order.

X-ray diffraction analysis was performed for the Al electrode layer 4thus formed. The result obtained was equivalent to that shown in theX-ray diffraction pole figure obtained in the second example.

As can be seen from FIG. 5, the twelve diffraction spots thus detectedshown in FIG. 4 were two sets of six-fold symmetry spots. The angulardistance between each symmetry spot and the corresponding symmetrycenter in the ψ direction was approximately 55°, and each symmetriccenter approximately coincided with the [111] direction of Al. Inaddition, the symmetry center and the normal line (the center of thepole figure) of the piezoelectric substrate 2 were spaced apart fromeach other by about ±10 to 20° in the ψ direction. In FIG. 5, thedistance between the symmetry center and the center of pole figure wasapproximately 17° in the ψ direction. However, since this angle variesin accordance with conditions of the piezoelectric substrate 2, afilm-forming temperature, or the like, in consideration of thisvariation, the distance can be considered to be in the range of about±10 to 20° as described above.

In the pole figure in the case of X-rays being incident on the Al(200)plane, the fact that the six-fold symmetry spots were observed meansthat the [111] direction of Al was oriented so as to approximatelycoincide with the center of the symmetry center, that is, a triaxialorientation film was formed in which the Al[111]-oriented film was grownin the direction toward the symmetry center.

In addition, the six-fold symmetry spots in themselves mean theformation of a twin. The reason for this is that when X-rays areincident on the Al(200) plane, the diffraction spots from the Al(111)single crystal are detected as the three-fold symmetry spots eachlocated at positions apart from the symmetry center by approximately55°. More particularly, they are three symmetry spots, i.e., (100),(010), and (001). The fact that the two sets of the three-fold symmetryspots were observed means that the two centers of the symmetry spotswere present, and that the Al[111]-oriented crystal had two growingdirections. In other words, the [111]-oriented single-crystal Al grew intwo orientation directions, that is, the twin structure was formed. Ascan be seen in FIG. 6, these two sets of the three-fold symmetry spotshave a positional relationship with each other in which one set islocated at a position rotated by 180° from that of the other set.

As described above, in the sixth example, since there were the two setsof the six-fold symmetry spots having the twin structure as shown inFIG. 5, this Al electrode layer 4 having this film structure can also beregarded as “a twin formed of two twins” (four single domains arepresent). That is, it may be assumed that the two sets of the three-foldsymmetry spots shown in FIG. 3 each form a twin.

Although the very complicated diffraction spots are shown in FIG. 4, theAl electrode layer 4 obtained in the sixth example is also a triaxialorientation epitaxial film formed of the Al(111) oriented in two growingdirections and is essentially the same as that in the second example.

Next, the electrode 3 was formed into an interdigital shape using aphotolithographic technique and a dry etching technique, thereby formingthe surface acoustic wave filter as the surface acoustic wave device 1.In addition, a surface acoustic wave filter having an Al electrodelayer, which was a uniaxial orientation film, was also formed as acomparative example in which the Al(111) plane grew perpendicular to thepiezoelectric substrate 2. When the surface acoustic wave filters thusformed were compared to each other, the electrical power resistance ofthe surface acoustic wave filter formed in the sixth example was 1,000times or more than that of the comparative example.

In the first to the sixth examples described above, as the piezoelectricsubstrate 2, a 36° Y-cut LiTaO₃ substrate and a 42° Y-cut LiTaO₃substrate are preferably used. However, in addition to those examples ofpreferred embodiments described above, any substrate formed of apiezoelectric single crystal may be used. In particular, the cut angleof the Y-cut is preferably in the range of from 36° to 42°. In addition,as the piezoelectric single crystal material for the piezoelectricsubstrates, a single crystal formed of LiTaO₃ or LiNbO₃ is preferablyused.

In addition, in the first to the sixth examples described above, as thematerial for the underlying electrode layer 5, Ti is preferably used.However, it has been confirmed that when another metal, such as Cr or analloy primarily composed of Cr or Ti, having an effect of improving thecrystallinity of the Al electrode layer 4 is used, the same advantage asdescribed above can be obtained.

In the first to the sixth examples described above, when the underlyingelectrode layer 5 is formed, a film-forming temperature therefor ispreferably set to about 180° C. However, the temperature is not limitedthereto. It has been confirmed that when this film-forming temperatureis changed, spot intensity of X-ray diffraction pole figure obtainedwhen the X-rays are incident on the Al(200) plane varies as shown inFIG. 7.

As can be seen from FIG. 7, when the film-forming temperature for theunderlying electrode layer 5 is about 70° C. or more, diffraction spotsof the Al electrode layer 4 formed thereon are detected, and anepitaxial Al film having the twin structure can be formed. However, inorder to obtain higher diffraction intensity, that is, to obtain an Alfilm having superior crystallinity, the Ti film-forming temperature forforming the underlying electrode layer 5 is preferably set to a highertemperature.

When the Ti film-forming temperature is excessively increased, due topyroelectric properties, since the piezoelectric substrate 2 tends to beeasily broken, the film-forming temperature is preferably about 300° C.or less in practice.

In addition, in preferred embodiments and the examples described above,the electrode layer is preferably primarily formed of Al. However, whena metal is used which has a face-centered cubic lattice and is primarilycomposed of platinum (Pt), gold (Au), Cu, or silver (Ag), the sameadvantages as those described above can be obtained.

As has thus been described, according to the surface acoustic wavedevice of preferred embodiments of the present invention, since theelectrode layer, which defines the electrode provided on thepiezoelectric substrate composed of a piezoelectric single crystal, isan orientation film formed by epitaxial growth and is also apolycrystalline thin film having a twin structure in which a diffractionpattern observed in an X-ray diffraction pole figure has a plurality ofsymmetry centers, the formation of hillocks and/or voids in theelectrode is prevented, and in addition, the plastic deformation thereofis prevented from occurring, thereby improving the electrical powerresistance of the surface acoustic wave device.

As the material for the electrode layer described above, when Al havinglow resistance and a low specific gravity is used, superior filterproperties can be obtained.

In the surface acoustic wave device described above, when the underlyingelectrode layer primarily composed, for example, of at least one of Tiand Cr is provided between the electrode layer and the piezoelectricsubstrate, the crystallinity of the electrode layer can be furtherimproved.

According to preferred embodiments of the present invention, the methodfor manufacturing a surface acoustic wave device further includes a stepof forming the intermediate electrode layer on the underlying electrodelayer at a low temperature of 50° C. or less in order to place a crystalface present on the surface of the underlying electrode layer in acleaner state, and the Al electrode layer is formed on this intermediateelectrode layer. Hence, even when the underlying electrode layer isprovided with an oxide layer thereon since being formed at an elevatedtemperature, a cleaner crystal face for forming the Al electrode layercan be obtained by the presence of the intermediate electrode layerwhile high orientation properties are maintained, thereby furtherimproving the crystallinity of the Al electrode layer.

According to preferred embodiments of the present invention, when aLiNbO₃ or a LiTaO₃ single crystal is used for the piezoelectricsubstrate, the piezoelectric property can be improved, and when thesurface acoustic wave device constitutes a filter or other device, theusable bandwidth therefor can be increased.

In addition, as the piezoelectric substrate, when a θ rotation Y-cut(θ=36° to 42°) LiTaO₃ substrate is used, an electrode layer having theparticular crystal structure described above can be reliably and easilyformed.

According to the method of preferred embodiments of the presentinvention for manufacturing a surface acoustic wave device, prior to thestep of forming the underlying electrode layer, the step for performingthe etching treatment for the piezoelectric substrate is performed toexpose a crystal face on the surface thereof so that the Al electrodelayer can be grown by epitaxial growth. Hence, when the underlyingelectrode layer is formed on the surface of the piezoelectric substratefollowed by the formation of the Al electrode layer, information oncrystal alignment, which is necessary for epitaxial growth, can besecurely transmitted to the Al electrode layer.

In addition, according to the method of preferred embodiments of thepresent invention for forming a surface acoustic wave device, when theelectrode is formed which includes the Al electrode layer primarilycomposed of Al and the underlying electrode layer provided between theAl electrode layer and the piezoelectric substrate for improving thecrystallinity of the Al electrode layer, the underlying electrode layeris formed by heating to a temperature of about 70° C. or more, andsubsequently, the Al electrode layer is formed at a low temperature ofabout 50° C. or less.

Accordingly, since energy required for crystal growth of the underlyingelectrode layer is supplied by heating, an underlying electrode layerhaving high orientation is obtained. Hence, the crystallinity of the Alelectrode layer formed on this underlying electrode layer is greatlyimproved, the stress migration resistance of the electrode is greatlyimproved, and the electrical power resistance of the surface acousticwave device thus formed is greatly improved.

In addition, when the Al electrode layer is formed, since film formationat a low temperature of about 50° C. or less is performed, the counterdiffusion between the Al and a material included in the underlyingelectrode layer can be avoided, and hence degradation of the Alelectrode layer caused by this counter diffusion can be prevented.

In the step of forming the underlying electrode layer by heating, when atemperature of about 300° C. or less is used, breakage of thepiezoelectric substrate caused by pyroelectric properties can besecurely prevented.

In addition, in the step of forming the Al electrode layer at a lowtemperature, when a temperature of about 0° C. or more is used, aspecific cooling apparatus is not necessary, and hence increase in costthereby can be avoided.

It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the present invention. Accordingly, the present inventionis intended to embrace all such alternatives, modifications andvariances which fall within the scope of the appended claims.

What is claimed is:
 1. A surface acoustic wave device comprising: apiezoelectric substrate made of one of a LiNbO₃ single crystal and aLiTaO₃ single crystal; and at least one electrode provided on thepiezoelectric substrate, wherein the at least one electrode includes anoriented electrode layer which is formed by epitaxial growth, saidoriented electrode layer being a polycrystalline thin film having a twinstructure in which a diffraction pattern observed in an X-raydiffraction pole figure has a plurality of symmetry centers.
 2. Asurface acoustic wave device according to claim 1, wherein the electrodelayer includes Al as a primary component.
 3. A surface acoustic wavedevice according to claim 1, wherein the electrode further comprises anunderlying electrode layer provided between the oriented electrode layerand the piezoelectric substrate for improving the crystallinity of theoriented electrode layer.
 4. A surface acoustic wave device according toclaim 3, wherein the underlying electrode layer includes at least one ofTi and Cr as a primary component.
 5. A surface acoustic wave deviceaccording to claim 3, wherein the electrode further comprises anintermediate electrode layer between an Al electrode layer and theunderlying electrode layer so as to cause a crystal face present on thesurface of the underlying layer to be in a cleaner state.
 6. A surfaceacoustic wave device according to claim 1, wherein the piezoelectricsubstrate is a θ rotation Y-cut LiTaO₃ substrate in which θ is in therange of from about 36° to about 42°.
 7. A surface acoustic wave deviceaccording to claim 1, wherein, when X-rays are incident on the (200)plane of the crystal constituting the electrode layer, the [111]direction of the crystal is oriented so as to approximately coincidewith the center of symmetry spots detected in the X-ray diffraction polefigure.
 8. A surface acoustic wave device according to claim 1, wherein,when X-rays are incident on the (200) plane of the crystal constitutingthe electrode layer, symmetry spots having at least two centers areobserved in the X-ray diffraction pole figure, the crystal of theelectrode layer grows in at least two [111] directions, and the [111]directions of the crystal are oriented so as to approximately coincidewith the centers of the symmetry spots detected in the X-ray diffractionpole figure.
 9. A surface acoustic wave device according to claim 8,wherein the symmetry spots detected in the X-ray diffraction pole figureform one of a three-fold symmetry and a six-fold symmetry.